Most synthetic plastics materials are derived from the raw polyolefins which are byproducts of the catalytic cracking of crude oil and the production of oil from coal. Raw polyolefins are in the form of powders comprising particles and lumps of various sizes. Chemical companies such as BASF and Bayer polymerise the raw polyolefins into a range of synthetic polymers usually without any fillers. Polymerisation occurs as a result of the application of heat and pressure. The nature of the resultant polymer depends on the degree to which the raw polyolefin is polymerised. Companies, which are often simply referred to as “compounders”, purchase this raw material and mix it with fillers such as talc, flame retardants, pigments and fibres. This is usually done by re-melting the raw material in a heated barrel which has two extruder screws therein. The two screws are parallel and side by side in the barrel and there can be one or more feed hoppers which feed fillers to the barrel. The screws mix the fillers and polymer as well as raise the temperature of the materials by a kneading action as the screws rotate.
The mixed, molten material which emerges from the barrel is fed as a rope through a cooling bath and then to a chopper that cuts the rope into pellets. The pellets are cut into lengths of up to about 25 mm. If the pellets have fibres in them, the pellet length and the maximum fibre length are substantially the same. This process results in plastics material pellets which contain the requisite additives.
For other uses the mixed molten material is fed through a silt like nozzle and extruded into the form of a continuous web. The web passes through a bath where it cools and sets. The web is then cut into sheets.
If the material is in pellet form it is bagged and shipped to the end user. End users are often referred to as converters. Most of the pellets produced as described above are used by feeding them to an injection moulding machine.
The first injection-moulding machine patent known to applicant was granted in the US in 1872 to John Hyatt. Almost three-quarters of a century later a major development occurred when William H Wilbert developed the reciprocating screw plasticiser for injection moulding machines. The patent was granted in 1956, injection moulding is principally a mass production method due to the required capital investment in machines, moulds and auxiliary equipment.
Before the advent of injection moulding compression moulding was the most important processing method for synthetic plastic materials. By 1960 the major processes in the plastics industry were injection moulding and extrusion. Twenty years later a wide variety of methods existed but injection moulding remained the dominant mass production technology for synthetic plastic components.
In the injection moulding process a synthetic plastics material, normally in pellet form, is added to the injection unit where it is subjected to the mixing and shearing action of a screw to provide a molten homogeneous mix. The mould is closed by a clamping unit. After complete closure of the mould the molten material in the injection unit is pushed forward through a sprue, a runner system and one or more gates into the mould cavity until the cavity is filled. The injection unit maintains pressure on the material whilst the material in the mould cavity cools and the material at the gate(s) solidifies. At that stage the plastisicing process re-commences and the screw moves back to the position it occupied before injection. Plastisicing, metering and injection are all carried out by the injection unit. Because the mould is fully closed when the molten material is injected, components with holes, undercuts etc can be made.
The process is characterized by the high clamping force required to keep the mould closed during filling of the mould cavity. The pressure can be reduced significantly by increasing the gate size but this increases the cycle time and the “witness” mark at the injection point.
To reduce the clamping force required, the moulds are in some machines kept open slightly. The material is then injected into the mould at a lower pressure and a lesser force is required to close the mould and complete the injection cycle. This method is referred to as injection compression moulding. Although the clamping pressure requirement is reduced, the process is limited to components without holes and undercuts. The shape of the components is limited by the relative movement of the two mould halves in the opening and closing direction. Product features dependent on the movement of mould components in a third dimension cannot be incorporated.
A further inherent limitation of the injection moulding process is in the moulding of long fibres into the product being made. The length of the pellet feedstock limits the initial length of the fibres. The average length of fibres in the material is further reduced by the plasticising process, by the high-pressure flow in the runners and by entry of the material into the mould cavity through the gate. Furthermore, the addition of fibres to the mouldable material decreases the flowability of the material. This significantly increases the clamping force requirements of the machine as injection at higher pressures is necessary to cause the material to flow. In addition, the abrasive action of fibres forced at high pressure through small passages significantly increases wear. It is for this reason that components requiring long fibres for strength are manufactured by processes such as the compression moulding process.
A significant difficulty with the injection moulding of certain articles is that the extraneous piece of material that comes out of the mould with the article, and which solidified in the sprue, must be trimmed-off. Where the article is of fibre filled material this cannot always be done by hand as the material is too hard. Hence machinery must be provided for removing the extraneous material.
The area around a sprue usually requires more time for cooling purposes than the rest of the article and this increases cycle time. Furthermore stresses and weakened zones can occur around the sprue.
Recent improvements in the productivity and cost effectiveness of the injection moulding process include the addition of a compounder. The compounder melts and mixes the mouldable material. The compounder feeds a number of “injection pots” each associated with clamping unit and mould. The injection pot comprises a barrel with a piston in it, the piston being reciprocable in the barrel. The barrel is closed at one end apart from a narrow sprue which leads to a runner system and then to the gate or gates at the mould cavity entrance(s). The molten material is transported from the compounder to the barrel of the injection pot by external hot runners. A valve at the entrance to the injection pot closes during the injection cycle. During the injection cycle the piston pushes a predetermined proportion of the molten material in the barrel through the sprue, runner system and gate(s). The pressure requirements remain the same as in injection moulding since no changes have been made to the way in which the material flows into the mould. Fibre breakage in the sprue, runner system and gate(s) is still present. There is a residual amount of material in the barrel at the sprue end after the injection part of the cycle.
The pellets can also be used in what is known as predetermined weight (or volume) compression moulding. In this form a slug or predetermined weight of molten material is placed into an open mould. The clamping pressure exerted when the mould is closed forces the molten mouldable material to spread out and fill the mould cavity. This method has the disadvantage that it cannot make an article with complex geometry and it is not possible to form either holes or undercuts in the article being moulded.
Compounders have lately also made their way into the compression moulding market. Long fibre-reinforced material is compounded, weighed and then placed into an open mould by a robot. The mould is closed and the component is formed by the mould halves which clamp the material between them. Material which when hot is sensitive to light, air or humidity cannot be moulded in this way as it is exposed to the atmosphere before it reaches the mould. Also, the shapes of the products produced are limited as in the case of injection compression moulding.
Sheets produced as described above can be used in processes such as vacuum and thermo forming or sheet web moulding which is another type of compression moulding. The heated sheet is placed in the mould whilst the mould is open, and the mould then closed to deform the soft sheet to the required shape. The resultant product, after cooling and solidifying, is known as a blank. Only simple shapes can be made by this method. Should holes be required in the part being manufactured then these are subsequently stamped out in a press. The blank must be positioned exactly in the press to ensure that the holes are in the correct place. There are normally trimming and finishing requirements on the stamped blank. This is particularly necessary if the sheet is fibre reinforced as stray fibres are usually left protruding from the cut edges. The pieces stamped out are normally recycled.
Compression moulding using sheet material cannot be employed for what is known as “In mould skin decoration”. This technique involves placing a layer of fabric a layer of paint skin or a layer of another material such as a glass fibre mat in the mould and moulding plastics material onto the back of it. The moulded-on material carries the layer and imparts the necessary strength to it. The difficulty which arises is that as the mould closes the sheet deforms and shifts, displacing the layer out of its intended position and possibly causing it to wrinkle.
Injection moulding has been employed to achieve in-mould decoration. However the high pressure of the incoming material can cause burn-through and can also cause the decorative layer to shift. The decorative layer has to be made thicker in order to avoid these difficulties.
It will be understood that energy is used when the compounding company melts the raw olefins to form the pellets or web. Further energy is used to melt the pellets to create a molten mass that can be moulded or to heat the sheet so that it can be formed to the requisite shape.
In the production of ceramic articles a green body is formed which is then sintered to achieve hardness and stability of shape. The mouldable material comprises the clay itself and a number of additives including water which enhance the properties of the raw clay and permit it to be moulded.
The additives and clay can be mixed in a compounder, with or without heating.
Techniques involving moulding using metallic powders mixed with a binder are in the process of development. These result in metal products which can be porous in nature.
The present invention seeks to provide an installation for, and a method of, manufacturing moulded articles which overcome shortcomings of the moulding and forming methods discussed above.